A joint within the human body forms a juncture between two or more bones or other skeletal parts. The ankle, hip, knee, shoulder, elbow and wrist are just a few examples of the multitude of joints found within the body. As should be apparent from the above list of examples of joints, many of the joints permit relative motion between the bones. For example, the motion of sliding, gliding, and hinge or ball and socket movements may be incorporated into a joint. For example, the ankle permits a hinge movement, the knee allows for a combination of gliding and hinge movements and the shoulder and hip permit movement through a ball and socket arrangement.
The joints in the body are stressed or can be damaged in a variety of ways. For example, the gradual wear and tear is imposed on the joints through the continuous use of a joint over the years. The joints that permit motion have cartilage positioned between the bones providing lubrication to the motion and also absorbing some of the forces direct to the joint. Over time, the normal use of a joint may wear down the cartilage and bring the moving bones in a direct contact with each other. In contrast, in normal use, a trauma to a joint, such as the delivery of a large force, from an accident for, example, an automobile accident, may cause considerable damage to the bones, the cartilage or to other connective tissue such as tendons or ligaments.
Arthropathy, a term referring to a disease of the joint, is another way in which a joint may become damaged. Perhaps the most well known joint disease is arthritis, which is generally referred to a disease or inflammation of a joint that results in pain, swelling, stiffness, instability, and often deformity.
There are many different forms of arthritis, with osteoarthritis being the most common and resulting from the wear and tear of a cartilage within a joint. Another type of arthritis is osteonecrosis, which is caused by the death of a part of the bone due to loss of blood supply. Other types of arthritis are caused by trauma to the joint while others, such as rheumatoid arthritis, Lupus, and psoriatic arthritis destroy cartilage and are associated with the inflammation of the joint lining.
The hip joint is one of the joints that is commonly afflicted with arthropathy. The hip joint is a ball and socket joint that joins the femur or thighbone with the pelvis. The pelvis has a semispherical socket called the acetabulum for receiving a ball socket head in the femur. Both the head of the femur and the acetabulum are coated with cartilage for allowing the femur to move easily within the pelvis. Other joints commonly afflicted with arthropathy include the spine, knee, shoulder, carpals, metacarpals, and phalanges of the hand. Arthroplasty as opposed to arthropathy commonly refers to the making of an artificial joint. In severe cases of arthritis or other forms of arthropathy, such as when pain is overwhelming or when a joint has a limited range of mobility, a partial or total replacement of the joint within an artificial joint may be justified. The procedure for replacing the joint varies, of course, with the particular joint in question, but in general involves replacing a terminal portion of an afflicted bone with a prosthetic implant and inserting a member to serve as a substitute for the cartilage.
The prosthetic implant is formed of a rigid material that becomes bonded with the bone and provides strength and rigidity to the joint and the cartilage substitute members chosen to provide lubrication to the joint and to absorb some of the compressive forces. Suitable material for the implant include metals, ceramics, composites and metals, for example, a titanium alloy, a cobalt chromium alloy, and a stainless steel alloy. Suitable materials for cartilage substitutes include polyethylene. A cement may also be used to secure the prosthetic implant to the host bone.
A total hip replacement, for example, involves removing the ball shaped head of the femur and inserting a stem implant into the center of the bone, which is referred to as the medullary canal, or marrow of the bone. The stem implant may be cemented into the medullary canal or may have a porous coated surface for allowing the bone to heal directly to the implant.
The stem implant has a neck and a ball shaped head, which are intended to perform the same functions as a healthy femur's neck and a ball shaped head. The polyethylene cup is inserted into the acetabulum and has a socket for receiving the head on the stem implant.
The invention relates to a surgical instrument for releasing the press fit of a joint insert in a joint socket.
Joint inserts are often held by a cone-type press fit in joint sockets, for example, in hip joint sockets. In order to release such joint inserts from the joint socket again, it is either necessary to destroy the joint inserts or to provide special devices on the joint socket which enable ejection of the joint insert from the joint socket. For example, DE 295 16 473 U1 describes a screw arrangement on the joint socket with which the joint insert can be pressed out of the joint socket. However, this makes it necessary for the joint socket to be of appropriate design and for an opening to be left in the joint insert.
An acetabular hip prosthesis has various design alternatives within their design and construction. One such design alternative has to do with whether the hip prosthesis is constrained or unconstrained. Similarly, a hip prosthesis, including a ball attached to a stem, is inserted into the canal of the resected femur and a socket including a portion of a spherical pocket is secured to the acetabulum. The portion of the ball in contact with the acetabular liner may represent less than 50% or a hemisphere of the head. In such an arrangement the head may freely be positioned in the liner. Alternatively, the acetabular component may extend around the head of the femur for an amount greater than 180°. In such a configuration the head may not freely be removed from the acetabular component. Such a construction is defined as a constrained prosthesis.
Referring now to FIG. 3, a prior art unconstrained prosthesis is shown as prosthesis 10. The prosthesis 10 includes a stem 12 and a head 14. The stem 14 is placed within femur 2. The head 14 matingly fits with liner 16, which is fitted within cup 18. The cup 18 is secured to acetabulum 4 of the patient. The liner 16 and the head 14 define a contact area therebetween having an included angle β, which is less 180°. Since β is less than 180°, the head 14 may freely move in and out of the liner 16.
It should be appreciated that use of an unconstrained liner may make a dislocation of the hip possible for the patient. If excessive extension of the leg is made by the patient, the head 14 may slip from the liner 14 and a dislocation or a misplacement of the head 14 may occur. If a dislocation has occurred, discomfort may accompany such dislocation and a surgical procedure to relocate the hip may be necessary.
The unconstrained hip prosthesis 10 as shown in FIG. 3, does however have the advantage of increased range of motion. The range of motion of the hip prosthesis 10 may be defined by an angle α and θ that represents the motion between the hip stem 12 and the liner 16 that can occur.
To avoid problems of dislocation of the head of the femur, prostheses have been provide for increased anglar contact between the liner and the head of the prosthesis. Such prosthesis are called constrained prosthesis. Constrained prosthesis prevent the occurrence of dislocation of the head of the prosthesis.
Referring now to FIG. 4, a constrained prosthesis 10′ is shown. The prosthesis 10′ includes a stem 12′ similar to the stem in FIG. 3. The stem 12′ has a head 14′ secured thereto. The head 14 may be similar to the head 14′ OF FIG. 3. The prosthesis 10′ further includes a cap 18 to which a liner 16′ is secured. The cap 18′ is secured to acetabulum 4 of the patient. The stem 12′ is secured to femur 2 of the patient similarly.
The liner 16 is somewhat different than the liner 16 of the prosthesis 10 of FIG. 3. The liner 16′ is a constrained liner. In other words, the liner 16′ contacts the head 14′ of the prosthesis 10′ at a contact angle θ′ which is greater than 180°. Since the contact angle between the head 14′ and liner 16′ is greater than 180°, the dislocation of the hip prosthesis 10′ is much less likely.
The head 14′ may be placed in the liner 16′ utilizing different techniques. For example, the liner 16′ may include a series of slits in the distal end thereof, which permit the distal portion of the liner 16′ to open until the head 14′ is assembled. After the head 14′ is assembled into the liner 16,′ a constraining ring 20′ may be positioned on the liner 16′.
To prevent the liner 16′ from being separated from the cap 18′, the liner 16′ may include an additional feature to secure the liner 16′ to the cap 18′. For example, the cap and liner may include grooves 22′ and 24′ respectively, for receiving a snap ring 26′ to be secured there between.
During revision surgery it may be necessary for a liner Δ to be removed from the prosthesis and be replaced with a new liner. The cap and stem may remain in the patient. The liner, thus, may need to be removed from the cap.
To remove a liner from the cap of an unconstrained prosthesis 10 as shown in FIG. 3, various devices are available for removing the liner. For example, a suction cup may be utilized to remove the liner from the cap or the liner may simply fall out in that it is not permanently secured to the cap 18. Alternatively, a screw 28 may be utilized to separate the liner from the cap as shown in U.S. Pat. No. 5,282,864 to Noiles, et al.
The removal of a liner from the constrained prosthesis 10′ of FIG. 4 is more difficult. The snap ring makes the removal of the constrained liner 16′ quite troublesome.
Attempts have been made to remove the locking ring from the grooves of the liner and shell by contracting the locking ring and then lifting the liner from the shell. Attempts of contracting the locking ring have proved unsuccessful because the locking ring may be inaccessible and difficult to compress in situ with the prosthesis in the patient.
Another attempt at removing the locking ring, including driving screws through the liner in hopes that the locking mechanism would break and lift the liner from the shell. The reaction force of the screw and the shells may be sufficient to overcome the force of the locking mechanism.
Another attempt to remove the liner from the shell was to cut the liner from the shell. Tools that are designed to cut liners may not be deep enough to expose the locking ring. Also, the use of a tool to cut the liner may raise concerns of creating polyethylene debris in the incision, which may prove to be problematic in that such debris may contribute to osteolysis.
This invention relates to the surgical instrument for removing the liner from the shell in situ in a patient where the forces necessary to separate the liner from the shell may be quite large.